The system of signs used is an independent question from the level of driver education (which probably is generally lower in the USA than in most European countries though), which also is another question from whether people "are stupid".

The obvious benefit of the European style is that you don't need to speak the language in order to understand the signs, something that is clearly less important given the relative homogeneity of language in the USA.

But at least to me, the European road signs are also more quick to read. For example, the "only right turns" and "speed limit 60" are exactly the same shape and color in the USA, while in Europe there is more of a logical system (which also means you don't "memorize a bunch of symbols", you learn the "language" of the signs), such as blue disc means "mandatory", red circle means "prohibited", red triangle means "warning", etc.

The European signs also fare better in snow, very heavy rain or if they're really dirty, since the shape (and maybe some colour) is still visible.